The pressure, P, developed beneath a pneumatic tourniquet cuff is largely governed by the gas or fluid pressure π in the tourniquet, which is assumed to act normally to the local support surface. In many practical instances of pneumatic tourniquet use, P is assumed to be equal to π. However, there are situations where this assumption may not be valid. In such cases hazard conditions can arise due to discrepancies between the indicated cuff pressure, π, and the actual applied pressure P. For instance, the outer walls of some pneumatic cuffs have integrated stiffener layers designed to stabilize and restrict radial expansion of the cuff when fitted to a supporting limb. The overall cuff geometry may be linear with oval cross section. When cuffs with such geometry are wrapped onto a limb and inflated, the inner wall membranes will necessarily fold and crease to accommodate the reduced circumferential scale. Such folding and creasing complicates the local stress field which may have significant orientation and magnitude variation producing wrinkling and pinching of the support tissue. Contoured cuffs and cuffs matched to limb size have been developed that greatly reduce this and other risks that can arise with simpler cuff geometries. Nevertheless, the inner wall membranes of such cuffs may support residual shearing and torsional forces and these may contribute additional components to the pressure P actually applied to the supporting tissue which is not reflected in the tourniquet measured pressure π.
Similarly, an over-tight un-inflated cuff may result in tissue applied pressures, P, sufficient to occlude venous return while indicating zero inflation pressure, π. Accidental blockages of the fluid lines used with pneumatic tourniquet cuffs can give rise to indicated cuff pressures, π, which can be higher or lower, depending upon the circumstances of the blockage, than the pressure in the cuff and consequently be unrepresentative of the actual tourniquet applied pressure P.
The use of soft, wrinkle-free padding (eg, cotton cast padding, stockinette, bandage) between the patient's limb and the tourniquet cuff is recommended by some pneumatic tourniquet cuff manufacturers in order to reduce cuff induced shear forces which can constitute a significant injury risk to soft tissue. However, such interventions may produce an offset or residual non-pneumatic pressure of unknown value resulting in a discrepancy between tourniquet pressure, π, and actual applied pressure, P. Again, such non-pneumatic pressures may be sufficiently high to restrict venous return from the supporting limb and impede the flow of arterial blood into the limb.
In non-pneumatic tourniquets commonly used in tactical and civilian emergency medical services (EMS) to restrict blood loss, the applied pressure is governed by the tension, T, in the tourniquet membrane or strap and the curvature, κ, of the membrane according to the law of Laplace, P=Tκ. In the case of membranes applied to cylindrical bodies, i.e. a membrane tourniquet on a limb, the form P=NT/r is frequently used by clinicians to estimate the applied pressure, where N is the number of complete wraps, r is the radius of curvature of the limb and the wrapping tension T is assumed to be constant. However, real limbs do not have regular geometric shapes and so the pressure will vary locally with curvature. In addition, the actual membrane tension may not be measured or known. Pressures calculated using the law of Laplace cannot be expected to accurately reflect the actual sub-tourniquet pressure at a given location on a limb or support tissue nor can such calculations provide a dynamic indication of the actual pressure applied by the tourniquet as limb position and orientation changes. A major criticism of such tourniquets used to restrict arterial blood flow, is that, if not properly applied, tourniquets can actually increase bleeding by occluding venous return while not completely arresting arterial flow {Doyle and Taillac, “Tourniquets: A Review of Current Use with Proposals for Expanded Prehospital Use”, Prehospital Emergency Care, V12, 2008, 241-54}.
The risk of tourniquet-related nerve injury continues to be a serious concern. Large sub-tourniquet pressure gradients have been identified as a significant contributory factor in a recent paper by Nordin et al. entitled “Surgical Tourniquets in Orthopaedics” published in The Journal of Bone & Joint Surgery (2009; 91: 2958-2967). The use of wider cuffs, where practicable, can reduce the size of such gradients.
In these and other biomedical applications it is desirable to have a transducer which may be located at the biomedical interface between the tourniquet and the limb which establishes the actual pressure applied by the tourniquet to the underlying support tissue, in a continuous way, throughout a medical procedure but also while the tourniquet is being applied to, and removed from the limb or tissue. The availability of such a transducer would greatly enhance patient safety and reduce the level of operator skill required in the routine use of tourniquets. Likewise, the availability of a reliable biomedical interface pressure gradient transducer would allow the development of enhanced tourniquet safety features designed to reduce the risk of nerve and deep tissue injury.
While there are biomedical pressure sensor devices available which may be placed between the tourniquet and the supporting limb or tissue, such sensors are necessarily intrusive and significant errors can arise due to the so called ‘hammocking-effect’ whereby a membrane such as bandage, cuff liner or indeed skin, stretches and curves in order to adjust to the shape of the sensor. In the case of a tourniquet on a limb, the tourniquet membrane lifts away from the supporting tissue in the vicinity of the sensor forming a profile somewhat like that of a hammock. The active sensor area is a critical parameter in the calibration of pressure sensors. It is normal to assume a constant active area based upon the geometric area of the sensor. However, the ‘hammocking-effect’ changes the effective area of the sensor which in turn varies with applied pressure/tension in a complex manner. Consequently, it is difficult to calibrate such sensors reliably for the wide range of tissue properties, curvatures and tourniquet membrane properties encountered in actual biomedical application environments.
Pneumatic tourniquets present further challenges in relation to the direct measurement of interface pressures generated when tourniquets are applied to limbs and tissue. As already described, the inner membranes of such tourniquets may buckle and fold and thereby support complex forces. Instead, normal and shear forces will co-exist and act on any sensor deployed at the interface. Pressure sensors are optimized for hydrostatic pressure measurement and so have optimum performance when the target pressure is relayed via fluid such as gas or liquid to the primary transduction element which is normally a cantilever or diaphragm. Without such a fluid, shear forces can result in large spurious pressure indications. Consequently, pressure measurement systems used in biomedical pressure measurement tend to use fluid lines to relay the pressure from the target measurement site to a sensor which is remote from the target environment (eg patent U.S. Pat. No. 4,584,625 disclosed by Kellogg describes a capacitive tactile sensor having gas-filled compression cells). However, the use of fluid lines has serious disadvantages for biomedical pressure applications. Fluid lines present a serious risk of error through partial or total obstruction due to folding and kinking of the lines. There is also a risk of fluid leak and contamination. Singly or combined these risks can present a significant safety risk to patients.
In WO2006030405, a low profile, flexible and planar biomedical pressure transducer is disclosed which greatly reduces the influence of the hammocking-effect on indicated pressures. This device is optimized for use as an interface pressure sensor for relatively low pressure applications such as bandaging and compression therapy. Such devices are not suitable for tourniquet applications where applied pressures may be as high as 500 mmHG and the interface region is subject to complex force fields.
U.S. Pat. No. 4,869,265 describes a biomedical pressure transducer having a pressurizable chamber containing integral membrane switch type electrical contacts interposed between the tissue and an apparatus such as a tourniquet cuff. The normally closed contacts are opened when the pressure within the chamber is equal to the pressure applied by the cuff. This approach does not appear to provide a continuous dynamic estimate of the pressure applied and suffers from signal damping introduced by the fluid line. In addition, this device appears to be complex.
In a planar transducer disclosed in U.S. Pat. No. 6,636,760 a structured elastomer element is used in a fiber optic based semi-rigid sensor suitable for use in the measurement of pressures under surgical tourniquet cuffs. Component alignment is critically important for such devices. This complicates the fabrication process and results in devices which are substantially rigid and intrusive requiring frequent calibration checking
U.S. Pat. No. 6,526,043 (Boie et al) describes a tactile transducer having a planar deformable layer and multiple conductive elements on each side. The arrangement is complex requiring sophisticated processing in order to infer precise pressure values.
Of the various electrical device properties which may be exploited in pressure transducers, electrical resistance is the easiest one to measure precisely over a wide range at moderate cost. Microelectromechanical system (MEMS) pressure sensor devices are widely available for general purpose, low cost, pressure measurement. These devices are typically piezoresistive, transforming a mechanical stress into a resistance signal which may be further transformed, through suitable electronic processing, into a pressure signal. Accordingly, this invention is directed towards providing an improved transducer for measuring biomedical interface pressures and pressure gradients at any interface between a human limb or tissue and a pneumatic or non-pneumatic tourniquet, using low cost, pre-calibrated, dynamic MEMS pressure sensors as the active pressure sensing element.
The applicant is aware of the following published references which are more or less relevant to the subject matter of the applicant's invention: V. Casey, S. Griffin and S. B. G. O'Brien, “An investigation of the hammocking effect associated with interface pressure measurements using pneumatic tourniquet cuffs”, Medical Engineering & Physics, V23, 2001, pp. 513-519; S. B. G. O'Brien and V. Casey, “Asymptotic and numerical solutions for a hammocking model”, Quarterly Journal of Mechanics and Applied Mathematics, V55, 2002, pp. 409-420; Intersema Application Note AN401, entitled Analog Sensor Interfacing.